Method of transferring a light emitting diode

ABSTRACT

A micro light emitting diode (LED) and a method of forming an array of micro LEDs for transfer to a receiving substrate are described. The micro LED structure may include a micro p-n diode and a metallization layer, with the metallization layer between the micro p-n diode and a bonding layer. A conformal dielectric barrier layer may span sidewalls of the micro p-n diode. The micro LED structure and micro LED array may be picked up and transferred to a receiving substrate.

RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 61/561,706 filed on Nov. 18, 2011 and U.S.Provisional Patent Application Ser. No. 61/594,919 filed on Feb. 3,2012, the full disclosures of which are incorporated herein byreference.

BACKGROUND

1. Field

The present invention relates to micro semiconductor devices. Moreparticularly embodiments of the present invention relate to a method offorming an array of micro devices such as light emitting diodes (LEDs)for transfer to a different substrate.

2. Background Information

Light emitting diodes (LEDs) based upon gallium nitride (GaN) areexpected to be used in future high-efficiency lighting applications,replacing incandescent and fluorescent lighting lamps. Current GaN-basedLED devices are prepared by heteroepitaxial growth techniques on foreignsubstrate materials. A typical wafer level LED device structure mayinclude a lower n-doped GaN layer formed over a sapphire growthsubstrate, a single quantum well (SQW) or multiple quantum well (MWQ),and an upper p-doped GaN layer.

In one implementation, the wafer level LED device structure is patternedinto an array of mesas on the sapphire growth substrate by etchingthrough the upper p-doped GaN layer, quantum well layer, and into then-doped GaN layer. An upper p-electrode is formed on the top p-doped GaNsurfaces of the array of mesas, and an n-electrode is formed on aportion of the n-doped GaN layer which is in contact with the array ofmesas. The mesa LED devices remain on the sapphire growth substrate inthe final product.

In another implementation, the wafer level LED device structure istransferred from the growth substrate to an acceptor substrate such assilicon, which has the advantage of being more easily diced to formindividual chips than a GaN/sapphire composite structure. In thisimplementation, the wafer level LED device structure is permanentlybonded to the acceptor (silicon) substrate with a permanent bondinglayer. For example, the p-electrode formed on the p-doped GaN surfacesof the array of mesas can be bonded to the acceptor (silicon) substratewith a permanent bonding layer. The sapphire growth substrate is thenremoved to expose the inverted wafer level LED device structure, whichis then thinned to expose the array of mesas. N-contacts are then madewith the exposed n-doped GaN, and p-contacts are made on the siliconsurface which is in electrical contact with the p-electrode. The mesaLED devices remain on the acceptor substrate in the final product. TheGaN/silicon composite can also be diced to form individual chips.

SUMMARY OF THE INVENTION

A micro light emitting diode (LED) and a method of forming an array ofmicro LEDs for transfer to a receiving substrate are described. Forexample, the receiving substrate may be, but is not limited to, adisplay substrate, a lighting substrate, a substrate with functionaldevices such as transistors or integrated circuits (ICs), or a substratewith metal redistribution lines. In an embodiment, a micro LED structureincludes a micro p-n diode and a metallization layer, with themetallization layer between the micro p-n diode and a bonding layerformed on a substrate. The metallization layer may include one or morelayers. For example, the metallization layer may include an electrodelayer and a barrier layer between the electrode layer and the bondinglayer. The micro p-n diode and metallization layer may each have a topsurface, a bottom surface and sidewalls. In an embodiment, the bottomsurface of the micro p-n diode is wider than the top surface of themicro p-n diode, and the sidewalls are tapered outwardly from top tobottom. The top surface of the micro p-n diode may also be wider thanthe bottom surface of the p-n diode, or approximately the same width. Inan embodiment, the bottom surface of the micro p-n diode is wider thanthe top surface of the metallization layer. The bottom surface of themicro p-n diode may also be wider than the top surface of themetallization layer, or approximately the same width as the top surfaceof the metallization layer.

A conformal dielectric barrier layer may optionally be formed over themicro p-n diode and other exposed surfaces. The conformal dielectricbarrier layer may be thinner than the micro p-n diode, metallizationlayer and optionally the bonding layer so that the conformal dielectricbarrier layer forms an outline of the topography it is formed on. In anembodiment, the conformal dielectric barrier layer spans sidewalls ofthe micro p-n diode, and may cover a quantum well layer in the micro p-ndiode. The conformal dielectric barrier layer may also partially spanthe bottom surface of the micro p-n diode, as well as span sidewalls ofthe metallization layer. In some embodiments, the conformal dielectricbarrier layer also spans sidewalls of a patterned bonding layer. Acontact opening may be formed in the conformal dielectric barrier layerexposing the top surface of the micro p-n diode. The contact opening canhave a width which is greater than, less than, or approximately the samewidth as the top surface of the micro p-n diode. In one embodiment, thecontact opening has a width which is less than the width of the topsurface of the micro p-n diode, and the conformal dielectric barrierlayer forms a lip around the edges of the top surface of the micro p-ndiode.

In some embodiments the bonding layer may be formed of a material whichhas a liquidus temperature or melting temperature below approximately350° C., or more specifically below approximately 200° C. For example,the bonding layer may include indium, tin or a thermoplastic polymersuch as polyethylene or polypropylene. The bonding layer may belaterally continuous across the substrate, or may also be formed inlaterally separate locations. For example, a laterally separate locationof the bonding layer may have a width which is less than orapproximately the same width as the bottom surface of the micro p-ndiode or metallization layer.

In an embodiment, a micro LED array includes a plurality of locations ofa bonding layer on a carrier substrate, and a corresponding plurality ofmicro LED structures on the plurality of locations of the bonding layer.Each micro LED structure includes a micro p-n diode and a metallizationlayer with the metallization layer between the micro p-n diode and arespective location of the bonding layer. A conformal dielectric barrierlayer can be deposited on the micro LED array on the substrate, with theconformal dielectric barrier layer spanning sidewalls of each micro p-ndiode. The conformal dielectric barrier layer may also partially spanthe bottom surface of each micro p-n diode, and sidewalls of eachmetallization layer. A plurality of contact openings may be formed inthe conformal dielectric barrier layer exposing a top surface of eachmicro p-n diode in which each contact opening has a width which may begreater than, less than, or approximately the same width as the topsurface of each corresponding micro p-n diode.

The plurality of locations of the bonding layer may or may not belaterally separate from one another. In some embodiments, the pluralityof locations of the bonding layer are laterally separate and theconformal dielectric barrier layer spans sidewalls of each of theplurality of laterally separate locations of the bonding layer. In someembodiments, the substrate includes a respective plurality of pillars onwhich the plurality of locations of the bonding layer are formed. Forexample, each micro p-n diode may include a bottom surface which iseither approximately the same width as a top surface of a respectivepillar or wider than the top surface of f the respective pillar. Thepillars may also have a height which is greater than a respectivethickness of the locations of the bonding layer. In an embodiment, therespective height is at least twice the respective thickness.

A micro LED structure and micro LED array may be formed utilizingexisting heterogeneous growth technologies. In an embodiment a p-n diodelayer and metallization layer are transferred from a growth substrate toa carrier substrate. In accordance with embodiments of the invention,the p-n diode layer and the metallization layer may be patterned priorto or after transfer to the carrier substrate. Transferring the p-ndiode layer and the metallization layer to the carrier substrate mayinclude bonding the metallization layer to a bonding layer on thecarrier substrate. For example, the bonding layer may have a liquidustemperature or melting temperature below approximately 350° C., or morespecifically below 200° C. For example, the bonding layer may be formedof indium or an indium alloy. After patterning the p-n diode layer andthe metallization layer to form a plurality of separate micro p-n diodesand a plurality of separate locations of the metallization layer aconformal dielectric barrier layer is formed spanning the sidewalls ofthe plurality of separate micro p-n diodes. The conformal dielectricbarrier layer may form an outline of the topography onto which it isformed, and may be thinner than the micro p-n diodes and themetallization layer. For example, the conformal dielectric barrier layermay be formed by atomic layer deposition (ALD). The conformal dielectricbarrier layer may also be formed on a portion of the bottom surface ofeach separate micro p-n diode.

In an embodiment, the p-n diode layer and a patterned metallizationlayer including a plurality of separate locations of the metallizationlayer on the p-n diode layer are transferred from the growth substrateto the carrier substrate. The p-n diode layer may be partially patternedprior to transferring from the growth substrate to the carriersubstrate, to form micro mesas separated by trenches in the p-n diodelayer. In an embodiment, a plurality of pillars are formed on thecarrier substrate prior to transferring the p-n diode layer andpatterned metallization layer to the carrier substrate. The bondinglayer may be formed over the plurality of pillars on the carriersubstrate prior to transferring the p-n diode layer and the patternedmetallization layer to the carrier substrate.

In an embodiment, the metallization layer is patterned to form aplurality of separate locations of the metallization layer aftertransferring the metallization layer and the p-n diode layer from thegrowth substrate to the carrier substrate. In such an embodiment, thep-n diode layer is patterned to form a plurality of separate micro p-ndiodes, followed by patterning the metallization layer. Patterning ofthe metallization layer may include etching the metallization layeruntil a maximum width of the plurality of separate locations of themetallization layer are less than a width of the bottom surface of eachof the plurality of separate micro p-n diodes. In an embodiment, thebonding layer is patterned after transferring the p-n diode layer andthe metallization layer form the growth substrate to the carriersubstrate. For example, the bonding layer can be etched until a maximumwidth of the plurality of separate locations of the bonding layer areless than a width of a bottom surface of each of the plurality ofseparate micro p-n diodes. A plurality of pillars can also be formed onthe carrier substrate prior to transferring the p-n diode layer and themetallization layer from the growth substrate to the carrier substrate.The bonding layer may be formed over the plurality of pillars on thecarrier substrate prior to transferring the p-n diode layer and thepatterned metallization layer to the carrier substrate.

Once formed, the micro LED structure and micro LED array can be pickedup and transferred to a receiving substrate. A transfer head can bepositioned over the carrier substrate having an array of micro LEDstructures disposed thereon, and an operation is performed to create aphase change in the bonding layer for at least one of the micro LEDstructures. For example, the operation may be heating the bonding layerabove a liquidus temperature or melting temperature of the bondinglayer, or altering a crystal phase of the bonding layer. The at leastone micro LED structure including the micro p-n diode and themetallization layer, and optionally a portion of the bonding layer forthe at least one of the micro LED structures may be picked up with atransfer head and placed on a receiving substrate. If a conformaldielectric barrier layer has already been formed, a portion of theconformal dielectric barrier layer may also be picked up with the microp-n diode and the metallization layer. Alternatively, a conformaldielectric barrier layer can be formed over the micro LED structure, orplurality of micro LED structures, after being placed on the receivingsubstrate.

In an embodiment, the conformal dielectric barrier layer spans a portionof the bottom surface of the micro p-n diode, spans sidewalls of themetallization layer, and spans across a portion of the bonding layeradjacent the metallization layer. The conformal dielectric barrier layermay be cleaved after contacting the micro LED structure with thetransfer head and/or creating the phase change in the bonding layer,which may be prior to picking up the micro p-n diode and themetallization layer with the transfer head. For example, cleaving theconformal dielectric barrier layer may include transferring a pressurefrom the transfer head to the conformal dielectric barrier layer and/orheating the bonding layer above a liquidus temperature of the bondinglayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional side view illustration of a bulk LEDsubstrate in accordance with an embodiment of the invention.

FIG. 1B is a cross-sectional side view illustration of a patternedmetallization layer in accordance with an embodiment of the invention.

FIG. 1C is a cross-sectional side view illustration of a patterned p-ndiode layer in accordance with an embodiment of the invention.

FIGS. 2A-2E are cross-sectional side view illustrations of a carriersubstrate with bonding layer in accordance with an embodiment of theinvention.

FIG. 3 is a cross-sectional side view illustration of bonding a growthsubstrate and carrier substrate together in accordance with anembodiment of the invention.

FIG. 4 is a cross-sectional side view illustration of various possiblestructures after bonding the growth substrate and carrier substratetogether in accordance with an embodiment of the invention.

FIG. 5 is a cross-sectional side view illustration of the growthsubstrate removed from the bonded structure in accordance with anembodiment of the invention.

FIG. 6 is a cross-sectional side view illustration of a thinned-down p-ndiode layer in accordance with an embodiment of the invention.

FIG. 7 is a cross-sectional side view illustration of etching p-n diodelayer to form micro p-n diodes in accordance with an embodiment of theinvention.

FIGS. 7′-7″ are a cross-sectional side view illustrations etching layersin accordance with an embodiment of the invention.

FIG. 8 is a cross-sectional side view illustration of various micro LEDstructures in accordance with an embodiment of the invention.

FIGS. 9-9′ are cross-sectional side view illustrations of the formationof contact openings in a micro LED array in accordance with anembodiment of the invention.

FIGS. 10-10″ are cross-sectional side view illustrations of theformation of contact openings in a micro LED array in accordance with anembodiment of the invention.

FIGS. 11A-11C are cross sectional side view illustrations of a wicked upbonding layer in accordance with an embodiment of the invention.

FIGS. 12A-12B include top and cross-sectional side view illustrations ofa carrier wafer and array of micro LED structures including micro p-ndiodes in accordance with an embodiment of the invention.

FIG. 13 is an illustration of a method of picking up and transferring amicro LED structure from a carrier substrate to a receiving substrate inaccordance with an embodiment of the invention.

FIG. 14 is a cross-sectional side view illustration of a transfer headpicking up a micro LED structure from a carrier substrate in accordancewith an embodiment of the invention.

FIG. 15 is a cross-sectional side view illustration of a bipolar microdevice transfer head in accordance with an embodiment of the invention.

FIG. 16 is a cross-sectional side view illustration of a receivingsubstrate with a plurality of micro LEDs in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe micro semiconductordevices and a method of forming an array of micro semiconductor devicessuch as micro light emitting diodes (LEDs) for transfer to a receivingsubstrate. For example, the receiving substrate may be, but is notlimited to, a display substrate, a lighting substrate, a substrate withfunctional devices such as transistors or integrated circuits (ICs), ora substrate with metal redistribution lines. While embodiments of thepresent invention are described with specific regard to micro LEDscomprising p-n diodes, it is to be appreciated that embodiments of theinvention are not so limited and that certain embodiments may also beapplicable to other micro semiconductor devices which are designed insuch a way so as to perform in a controlled fashion a predeterminedelectronic function (e.g. diode, transistor, integrated circuit) orphotonic function (LED, laser).

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “in anembodiment” or the like in various places throughout this specificationare not necessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “spanning,” “over,” “to,” “between” and “on” as used hereinmay refer to a relative position of one layer with respect to otherlayers. One layer “spanning,” “over” or “on” another layer or bonded“to” another layer may be directly in contact with the other layer ormay have one or more intervening layers. One layer “between” layers maybe directly in contact with the layers or may have one or moreintervening layers.

The terms “micro” device, “micro” p-n diode or “micro” LED structure asused herein may refer to the descriptive size of certain devices orstructures in accordance with embodiments of the invention. As usedherein, the terms “micro” devices or structures are meant to refer tothe scale of 1 to 100 μm. However, it is to be appreciated thatembodiments of the present invention are not necessarily so limited, andthat certain aspects of the embodiments may be applicable to larger, andpossibly smaller size scales.

In one aspect, embodiments of the invention describe a method ofprocessing a bulk LED substrate into an array of micro LED structureswhich are poised for pick up and transfer to a receiving substrate. Inthis manner, it is possible to integrate and assemble micro LEDstructures into heterogeneously integrated systems. The micro LEDstructures can be picked up and transferred individually, in groups, oras the entire array. Thus, the micro LED structures in the array ofmicro LED structures are poised for pick up and transfer to a receivingsubstrate such as display substrate of any size ranging from microdisplays to large area displays, and at high transfer rates. In someembodiments, arrays of micro LED structures which are poised for pick upare described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μm pitch.At these densities a 6 inch substrate, for example, can accommodateapproximately 165 million micro LED structures with a 10 μm by 10 μmpitch, or approximately 660 million micro LED structures with a 5 μm by5 μm pitch. Thus, a high density of pre-fabricated micro devices with aspecific functionality may be produced in a manner in which they arepoised for pick up and transfer to a receiving substrate. The techniquesdescribed herein are not limited to micro LED structures, and may alsobe used in the manufacture of other micro devices.

In another aspect, embodiments of the invention describe a micro LEDstructure and micro LED array in which each micro p-n diode is formedover a respective location of a bonding layer. The respective locationsof the bonding layer may or may not be laterally separate locations. Anoperation may be performed on a respective location of the bonding layercorresponding to a micro LED during the micro LED pick up process inwhich the respective location of the bonding layer undergoes a phasechange which assists in the pick up process. For example, the respectivelocation of the bonding layer may change from solid to liquid inresponse to a temperature cycle. In the liquid state the respectivelocation of the bonding layer may retain the micro p-n diode in place ona carrier substrate through surface tension forces, while also providinga medium from which the micro p-n diode is readily releasable. Inaddition, the liquid state may act as a cushion or shock absorber toabsorb forces exerted by a transfer head if a transfer head makescontact with the micro LED structure during the pick up process. In thismanner, the liquid state may compensate for non-uniformities in thetopography in the micro LED array or transfer head array by smoothingout over the underlying surface in response to compressive forcesexerted by a transfer head. In other embodiments, the respectivelocation of the bonding layer may not undergo a complete phasetransformation. For example, the respective location of the bondinglayer may become substantially more malleable in response to atemperature cycle while partially remaining in the solid state. Inanother embodiment, the respective location of the bonding layer mayundergo a crystal phase transformation in response to an operation, suchas a temperature cycle.

Referring now to FIG. 1, a semiconductor device layer 110 may be formedon a substrate 101. In an embodiment, semiconductor device layer 110 mayinclude one or more layers and is designed in such a way so as toperform in a controlled fashion a predetermined electronic function(e.g. diode, transistor, integrated circuit) or photonic function (LED,laser). It is to be appreciated that while semiconductor device layer110 may be designed in such a way so as to perform in a controlledfashion in a predetermined function, that the semiconductor device layer110 may not be fully functionalized. For example, contacts such as ananode or cathode may not yet be formed. In the interest of concisenessand to not obscure embodiments of the invention, the followingdescription is made with regard to semiconductor device layer 110 as ap-n diode layer 110 grown on a growth substrate 101 in accordance withconventional heterogeneous growth conditions.

The p-n diode layer 110 may include a compound semiconductor having abandgap corresponding to a specific region in the spectrum. For example,the p-n diode layer 110 may include one or more layers based on II-VImaterials (e.g. ZnSe) or III-V nitride materials (e.g. GaN, AN, InN, andtheir alloys). Growth substrate 101 may include any suitable substratesuch as, but not limited to, silicon, SiC, GaAs, GaN and sapphire(Al₂O₃).

In a particular embodiment, growth substrate 101 is sapphire, and thep-n diode layer 110 is formed of GaN. Despite the fact that sapphire hasa larger lattice constant and thermal expansion coefficient mismatchwith respect to GaN, sapphire is reasonably low cost, widely availableand its transparency is compatible with excimer laser-based lift-off(LLO) techniques. In another embodiment, another material such as SiCmay be used as the growth substrate 101 for a GaN p-n diode layer 110.Like sapphire, SiC substrates may be transparent. Several growthtechniques may be used for growth of p-n diode layer 110 such asmetalorganic chemical vapor deposition (MOCVD). GaN, for example, can begrown by simultaneously introducing trimethylgallium (TMGa) and ammonia(NH₃) precursors into a reaction chamber with the sapphire growthsubstrate 101 being heated to an elevated temperature such as 800° C. to1,000° C. In the particular embodiment illustrated in FIG. 1A, p-n diodelayer 110 may include a bulk GaN layer 112, an n-doped layer 114, aquantum well 116 and p-doped layer 118. The bulk GaN layer 112 may ben-doped due to silicon or oxygen contamination, or intentionally dopedwith a donor such as silicon. N-doped GaN layer 114 may likewise bedoped with a donor such as silicon, while p-doped layer 118 may be dopedwith an acceptor such as magnesium. A variety of alternative p-n diodeconfigurations may be utilized to form p-n diode layer 110. Likewise, avariety of single quantum well (SQW) or multiple quantum well (MQW)configurations may be utilized to form quantum well 116. In addition,various buffer layers may be included as appropriate. In one embodiment,the sapphire growth substrate 101 has a thickness of approximately 200μm, bulk GaN layer 112 has a thickness of approximately 5 μm, n-dopedlayer 114 has a thickness of approximately 0.1 μm-3 μm, quantum welllayer 116 has a thickness less than approximately 0.3 μm and p-dopedlayer 118 has a thickness of approximately 0.1 μm-1 μm.

A metallization layer 120 may then be formed over the p-n diode layer110. As illustrated in FIG. 1A, metallization layer 120 may include anelectrode layer 122 and optionally a barrier layer 124, though otherlayers may be included. In an embodiment, metallization layer has athickness of approximately 0.1 μm-2 μm. Electrode layer 122 may makeohmic contact to the p-doped GaN layer 118, and may be formed of a highwork-function metal such as Ni, Au, Ag, Pd and Pt. In an embodiment,electrode layer 122 may be reflective to light emission. In anotherembodiment, electrode layer 122 may also be transparent to lightemission. Transparency may be accomplished by making the electrode layervery thin to minimize light absorption. Barrier layer 124 may optionallybe included in the metallization layer 120 to prevent diffusion ofimpurities into the p-n diode 110. For example, barrier layer 124 mayinclude, but is not limited to, Pd, Pt, Ni, Ta, Ti and TiW. In certainembodiments, barrier layer 124 may prevent the diffusion of componentsfrom the bonding layer into the p-n diode layer 110.

In accordance with certain embodiments of the invention, p-n diode layer110 and metallization layer 120 are grown on a growth substrate 101 andsubsequently transferred to a carrier substrate 201, such as oneillustrated in FIGS. 2A-2E and described in more detail in the followingdescription. As described in more detail in the following figures anddescription, the metallization layer 120 and p-n diode layer 110 can bepatterned prior to transfer to a carrier substrate 201. The carriersubstrate 201 and bonding layer 210 may also be patterned prior totransfer of the p-n diode layer 110 and metallization layer 120 to thecarrier substrate 201. Accordingly, embodiments of the invention may beimplemented in a multitude of variations during formation of an array ofmicro LEDs for subsequent transfer to a receiving substrate.

Referring now to FIG. 1B metallization layer 120 may be patterned priorto transfer to a carrier substrate 201. In an embodiment, the structureof FIG. 1B may be achieved by forming a patterned photoresist layer overthe p-n diode layer 110 followed by deposition of the metallizationlayer 120. The photoresist layer is then lifted off (along with theportion of the metallization layer on the photoresist layer) leavingbehind the laterally separate locations of metallization layer 120illustrated in FIG. 1B. In certain embodiments, the pitch of thelaterally separate locations of metallization layer 120 may be 5 μm, 10μm, or larger corresponding to the pitch of the array of micro LEDs. Forexample, a 5 μm pitch may be formed of 3 μm wide laterally separatelocations of metallization layer 120 separated by a 2 μm spacing. A 10μm pitch may be formed of 8 μm wide separate locations of metallizationlayer 120 separated by a 2 μm spacing. Though, these dimensions aremeant to be exemplary and embodiments of the invention are not solimited. In some embodiments, the width of the laterally separatelocations of metallization layer 120 is less than or equal to the widthof the bottom surface of the array of micro p-n diodes 150 as discussedin further detail in the following description and figures.

Referring now to FIG. 1C patterning of the metallization layer 120 maybe followed by patterning of p-n diode layer 110. In an embodiment, thestructure of FIG. 1C may be achieved by forming a second patternedphotoresist layer over the laterally separate locations of metallizationlayer 120 and an etchant is applied to etch the p-n diode layer 110 toetch trenches 134 and form a plurality of micro mesas 130. Referringagain to the enlarged section of p-n diode layer 110 in FIG. 1A, in anembodiment, etching is performed to etch trenches through the p-dopedlayer 118, quantum well 116, and into the n-doped layer 114 or bulklayer 112. Etching of the GaN p-n diode layer 110 can be performedutilizing dry plasma etching techniques such as reactive ion etching(RIE), electro-cyclotron resonance (ECR), inductively coupled plasmareactive ion etching ICP-RIE, and chemically assisted ion-beam etching(CAIBE). The etch chemistries may be halogen-based, containing speciessuch as Cl₂, BCl₃ or SiCl₄. In the particular embodiment illustrated inFIG. 1C, micro mesas 130 may have tapered sidewalls 132 up to 15degrees. For example, RIE with a chlorine-based etch chemistry may beutilized. Alternatively, the sidewalls may be vertical. For example,ICP-RIE which a chlorine-based etch chemistry may be utilized to obtainvertical sidewalls.

In certain embodiments, the pitch of the micro mesas 130 may be 5 μm, 10μm, or larger. For example, a micro mesa 130 array with a 5 μm pitch maybe formed of 3 μm wide micro mesas separated by a 2 μm spacing. A micromesa 130 array with a 10 μm pitch may be formed of 8 μm wide micro mesasseparated by a 2 μm spacing. Though, these dimensions are meant to beexemplary and embodiments of the invention are not so limited.

FIGS. 2A-2E are cross-sectional side view illustrations of variousembodiments of a carrier substrate 201 with bonding layer 210 forbonding to the metallization layer 120 on growth substrate 101. FIG. 2Aillustrates a carrier substrate 201 and bonding layer 210 which are notpatterned prior to bonding. FIGS. 2B-2D illustrate a carrier substrate201 which has been patterned to form a plurality of posts 202 havingsidewalls 204 and separated by trenches 206. Posts 202 may have amaximum width which is equal to or less than a width of the micro p-ndiodes 135, 150, as will become more apparent in the followingdescription and figures. In an embodiment, the trench posts 202 are atleast twice as tall as a thickness of the bonding layer 210. In anembodiment, bonding layer 210 may have a thickness of approximately 0.1μm-2 μm, and trench posts have a height of at least 0.2 μm-4 μm. In theparticular embodiment illustrated in FIG. 2B, a conformal bonding layer210 is formed over the posts 202, and on the sidewalls 204 and withintrenches 206. In the particular embodiment illustrated in FIG. 2C,bonding layer 210 is anisotropically deposited so that it is formed onlyon the top surface of posts 202 and within the trenches 206, without asignificant amount being deposited on the sidewalls 204. In theparticular embodiment illustrated in FIG. 2D, bonding layer 210 isformed only on the top surface of posts 202. Such a configuration may beformed by patterning the posts 202 and bonding layer 210 with the samepatterned photoresist. In the particular embodiment illustrated in FIG.2E, the laterally separate locations of the bonding layer 210 may beformed with a photoresist lift off technique in which a blanket layer ofthe bonding layer is deposited over a patterned photoresist layer, whichis then lifted off (along with the portion of the bonding layer on thephotoresist layer) leaving behind the laterally separate locations ofthe bonding layer 210 illustrated in FIG. 2E, though other processingtechniques may be used.

As described above with regard to FIGS. 2B-2E and FIGS. 1B-1C, certainembodiments of the invention include laterally separate locations of themetallization layer 120 and/or laterally separate locations of thebonding layer 210. With regard to FIG. 2B, in which a conformal bondinglayer 210 is formed over the posts 202, and on the sidewalls 204 andwithin trenches 206, the particular locations of the bonding layer ontop of the posts 202 are laterally separated by the trenches 206. Thus,even though the conformal bonding layer 210 is continuous, the locationsof the bonding layer 210 on top of the posts 202 are laterally separatelocations Likewise, the individual discrete locations of the bondinglayer 210 in FIG. 2E are laterally separated by the space between them.Where posts 202 exist, the relationship of the bonding layer 210thickness to post 202 height may factor into the lateral separation ofthe locations of the bonding layer 210.

Bonding layer 210 may be formed from a variety of suitable materials.Bonding layer may be formed from a material which is capable of adheringa micro LED structure to a carrier substrate. In an embodiment, bondinglayer 210 may undergo a phase change in response to an operation such aschange in temperature. In an embodiment, bonding layer may be removableas a result of the phase change. In an embodiment, bonding layer may beremeltable or reflowable. In an embodiment, the bonding layer may have aliquidus temperature or melting temperature below approximately 350° C.,or more specifically below approximately 200° C. At such temperaturesthe bonding layer may undergo a phase change without substantiallyaffecting the other components of the micro LED structure. For example,the bonding layer may be formed of a metal or metal alloy, or of athermoplastic polymer which is removable. In an embodiment, the bondinglayer may be conductive. For example, where the bonding layer undergoesa phase change from solid to liquid in response to a change intemperature a portion of the bonding layer may remain on the micro LEDstructure during the pick up operation as described in more detail thefollowing description. In such an embodiment, it may be beneficial thatthe bonding layer is formed of a conductive material so that it does notadversely affect the micro LED structure when it is subsequentlytransferred to a receiving substrate. In this case, the portion ofconductive bonding layer remaining on the micro LED structure during thetransfer operation may aid in bonding the micro LED structure to aconductive pad on the receiving substrate.

Solders may be suitable materials for bonding layer 210 since many aregenerally ductile materials in their solid state and exhibit favorablewetting with semiconductor and metal surfaces. A typical alloy melts nota single temperature, but over a temperature range. Thus, solder alloysare often characterized by a liquidus temperature corresponding to thelowest temperature at which the alloy remains liquid, and a solidustemperature corresponding to the highest temperature at which the alloyremains solid. An exemplary list of low melting solder materials whichmay be utilized with embodiments of the invention are provided in Table1.

TABLE 1 Liquidus Temperature Solidus Temperature Chemical composition (°C.) (° C.) 100 In 156.7 156.7 66.3In33.7Bi 72 72 51In32.5Bi16.5Sn 60 6057Bi26In17Sn 79 79 54.02Bi29.68In16.3Sn 81 81 67Bi33In 109 109 50In50Sn125 118 52Sn48In 131 118 58Bi42Sn 138 138 97In3Ag 143 143 58Sn42In 145118 99.3In0.7Ga 150 150 95In5Bi 150 125 99.4In0.6Ga 152 152 99.6In0.4Ga153 153 99.5In0.5Ga 154 154 60Sn40Bi 170 138 100Sn 232 232 95Sn5Sb 240235

An exemplary list thermoplastic polymers which may be utilized withembodiments of the invention are provided in Table 2.

TABLE 2 Melting Temperature Polymer (° C.) Acrylic (PMMA) 130-140Polyoxymethylene (POM or Acetal) 166 Polybutylene terephthalate (PBT)160 Polycaprolactone (PCL)  62 Polyethylene terephthalate (PET) 260Polycarbonate (PC) 267 Polyester 260 Polyethylene (PE) 105-130Polyetheretherketone (PEEK) 343 Polylactic acid (PLA) 50-80Polypropylene (PP) 160 Polystyrene (PS) 240 Polyvinylidene chloride(PVDC) 185

In accordance with embodiments of the invention, bonding layer 210 isformed with a uniform thickness and may be deposited by a variety ofsuitable methods depending upon the particular composition. For example,solder compositions may be sputtered, deposited by electron beam(E-beam) evaporation, or plated with a seed layer to obtain a uniformthickness.

Posts 202 may be formed from a variety of materials and techniques. Inan embodiment, posts 202 may be formed integrally with carrier substrate201 by patterning the carrier substrate 201 by an etching or embossingprocess. For example, carrier substrate 201 may be a silicon substratewith integrally formed posts 202. In another embodiment, posts can beformed on top of carrier substrate 201. For example, posts 202 may beformed by a plate up and photoresist lift off technique. Posts can beformed from any suitable material including semiconductors, metals,polymers, dielectrics, etc.

Referring now to FIG. 3, the growth substrate 101 and carrier substrate201 may be bonded together under heat and/or pressure. It is to beappreciated that while FIG. 3 illustrates the bonding of the patternedstructure of FIG. 1B with the unpatterned structure of FIG. 2A, that anycombination of FIGS. 1A-1C and FIGS. 2A-2E are contemplated inaccordance with embodiments of the invention. In addition, while it hasbeen described that bonding layer 210 is formed on the carrier substrate201 prior to bonding, it is also possible that the bonding layer 210 isformed on the metallization layer 120 of the growth substrate 101 priorto bonding. For example, bonding layer 210 could be formed overmetallization layer 120, and patterned with metallization layer 120during formation of the laterally separate locations of metallizationlayer illustrated in FIG. 1B. While not illustrated, depending upon theparticular arrangement and composition of layers in formed on thesubstrates to be bonded together, an oxidation resistant film may beformed on the top surface of either or both substrates to preventoxidation prior to bonding. For example, in one embodiment, a thin goldfilm can be deposited on either or both of the exposed surface ofmetallization layer 120 and bonding layer 210. During bonding of thesubstrates illustrated in FIG. 3, the bonding layer 210 may partiallysoak up the gold film resulting in a gold alloy at the bonding interfacebetween the substrates.

FIG. 4 is a cross-sectional side view illustration of variousnon-limiting possible structures after bonding the growth substrate 101and carrier substrate 201. The particular combinations of substrates aredescribed in Table 3. For example, the particular embodiment illustratedin Example 4A represents the bonding of the carrier substrateillustrated in FIG. 2D to the growth substrate illustrated in FIG. 1C.

TABLE 3 Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. 4A4B 4C 4D 4E 4F 4G 4H 4I 4J 4K 4L 4M 4N 4O Carrier 2D 2C 2B 2D 2C 2B 2A2E 2A 2E 2D 2C 2B 2A 2E Substrate (2A-2D) Growth 1C 1C 1C 1A 1A 1A 1A 1A1C 1C 1B 1B 1B 1B 1B Substrate (1B)

As described above, the structures of many of the examples can also becreated by forming the bonding layer 210 on the growth substrate,followed by bonding the growth substrate 101 to the carrier substrate201. For example, example 4O, can also be created by patterning bondinglayer 210 and metallization layer 210 on growth substrate 101, followingby bonding the growth substrate 101 to carrier substrate 201.

Referring now to FIG. 5, the growth substrate 101 has been removed fromthe bonded structure. Growth substrate 101 may be removed by a suitablemethod such as chemical etching or an excimer laser-based lift-off (LLO)if the growth substrate is transparent. In an embodiment, LLO of a GaNp-n diode layer 110 from a transparent sapphire growth substrate 101 isaccomplished by irradiating the 101/110 layer interface through thetransparent sapphire growth substrate 101 with a short pulse (e.g. tensof nanoseconds) from an ultraviolet laser such as a Nd-YAG laser or KrFexcimer laser. Absorption in the GaN p-n diode layer 110 at theinterface results in localized heating of the interface resulting indecomposition at the interfacial GaN to liquid Ga metal and nitrogengas. Once the desired are has been irradiated, the transparent sapphiregrowth substrate 101 can be removed by remelting the Ga on a hotplate.

Referring now to FIG. 6, the p-n diode layer 110 is thinned down to adesirable thickness. Referring back to the enlarged p-n diode layer 110in FIG. 1A, a predetermined amount of the bulk GaN layer 112 (which maybe n-type) or a portion of the n-type GaN layer 114 are removed so thatan operable p-n diode remains after thinning. Depending upon theunderlying structure, the thinning process may be performed utilizingsuitable techniques such as polishing, wet etching or dry etching. Forexample, a combination of polish and/or timed etch to a desiredthickness may be performed. In circumstances where there are underlyingpatterned structures such as pillars or micro mesas, a timed etch to adesired thickness may be performed in order to avoid damaging thepatterned structures. As shown in Examples 6A, 6B, 6C, 6I and 6J wherethe p-n diode layers 110 were pre-patterned to form micro mesas 130,they are now free-standing micro p-n diodes 135.

If either of the growth substrate 101 or carrier substrate 201structures were not pre-patterned or only partially pre-patterned priorto bonding, then additional patterning may be performed after the p-ndiode layer 110 thinning illustrated in FIG. 6. As illustrated in FIG. 7a patterned mask layer 140 may be formed over the unpatterned p-n diodelayer 110 for etching of p-n diode layer 110 to form free standing microp-n diodes 150. Mask layer 140 may be formed from photoresist or avariety of materials such as metal (e.g. chromium, nickel) or dielectric(silicon nitride, silicon oxide) which are more resistant to the GaNetching conditions than is photoresist. Etching of the GaN p-n diodelayer 110 can be performed utilizing dry plasma etching techniques suchas reactive ion etching (RIE), electro-cyclotron resonance (ECR),inductively coupled plasma reactive ion etching (ICP-RIE), andchemically assisted ion-beam etching (CAIBE). The etch chemistries maybe halogen-based, containing species such as Cl₂, BCl₃ or SiCl₄.

In the particular embodiment illustrated in FIG. 7, micro p-n diodes 150may have outwardly tapered sidewalls 153 (from top to bottom of themicro p-n diodes 150) up to 15 degrees. For example, RIE with achlorine-based etch chemistry may be utilized. Alternatively, thesidewalls 153 may be vertical. For example, ICP-RIE which achlorine-based etch chemistry may be utilized to obtain verticalsidewalls. As will become apparent in the description of FIG. 16,outwardly tapered sidewalls may be advantageous in some embodiments whenforming a common contact over a series of micro LED structures whichhave been picked up and transferred to a receiving substrate. In certainembodiments, the pitch between the micro p-n diodes 150 may be 5 μm, 10μm, or larger. For example, a micro p-n diode 150 array with a 5 μmpitch may be formed of 3 μm wide micro p-n diodes separated by a 2 μmspacing. A micro p-n diode 150 array with a 10 μm pitch may be formed of8 μm wide micro p-n diodes separated by a 2 μm spacing.

Referring now to FIGS. 7′-7″, etching may optionally be continued onmetallization layer 120 and/or bonding layer 210 utilizing suitableetching chemistries based upon the particular materials in metallizationlayer 120 and bonding layer 210. In certain embodiments illustrated inFIG. 7′, anisotropic etching with a dry etching chemistry can beutilized to etch metallization layer 120 and/or bonding layer 210 sothat the layers 120, 210 have a width matching the overlying lowersurface of the micro p-n diode 150. In certain embodiments illustratedin FIG. 7′″, wet etching may be utilized to “undercut” the metallizationlayer 120 and/or bonding layer 210 underneath the overlying lowersurface of the micro p-n diode 150 as illustrated in Examples 7″D-7″H.While not specifically illustrated, it is understood that etching couldalso be performed to “undercut” the underlying layers 120, 210underneath micro p-n diodes 135.

Upon completion of etching processes for the micro p-n diodes,metallization layer or bonding layer, the mask layer 140 may be removed,for example by using a selective etching technique, resulting the microLED array illustrated in FIG. 8. As illustrated, the micro LED arrayincludes a carrier substrate 201, a plurality of locations of a bondinglayer 210 (which may or may not be laterally separate) on the carriersubstrate, and a respective plurality of separate micro p-n diodes 135,150 over the plurality of locations of the bonding layer 210. Aplurality of separate locations of metallization layer 120 are formedbetween the respective plurality of separate micro p-n diodes 135, 150and the plurality of locations of the bonding layer 210. In someembodiments, the carrier substrate includes a respective plurality ofpillars 202 on which the plurality of laterally separate locations ofthe bonding layer 210 are formed, as illustrated in Examples 8A-8F andExamples 8K-8M.

In some embodiments, the micro p-n diodes 150 (as well as micro p-ndiodes 135) include a top surface 152 and a bottom surface 151, and themetallization layer 120 includes a top surface 121 and a bottom surface,and the bottom surface 151 of the micro p-n diode 150 (as well as microp-n diodes 135) is wider than the top surface 121 of the metallizationlayer 120.

In some embodiments, the plurality of micro p-n diodes 135, 150 eachinclude a bottom surface 151 which has approximately the same width as atop surface 203 of each of the respective plurality of pillars 202. Inother embodiments, the plurality of micro p-n diodes 135, 150 eachinclude a bottom surface 151 which is wider than a top surface 203 ofeach of the respective plurality of pillars 202. The relationship of themicro p-n diode 135, 150 bottom width and underlying pillar 202 topsurface may affect the pick up process. For example, if the bondinglayer 210 exhibits a phase change from solid to liquid during the pickup process then the micro p-n diode 135, 150 is essentially floating ona liquid layer. Surface tension forces in the liquid bonding layer 210may retain the micro p-n diode 135, 150 in place on top of the pillar202. In particular, surface tension forces associated with the edges ofthe top surface of the pillar 202 may further assist in maintaining themicro p-n diode 135, 150 in place where the pillar 202 top surface widthis less than or approximately equal to the p-n diode 135, 150 bottomwidth.

In some embodiments, the plurality of micro p-n diodes 135, 150 arepositioned over an unpatterned bonding layer 210. For example, asillustrated in Example 6I and Example 7′N, the bonding layer 210 may bea uniform layer on the carrier substrate and the corresponding pluralityof locations of the bonding layer 210 are not laterally separate fromeach other. In other embodiments, the plurality of micro p-n diodes 135,150 are positioned over a pattered bonding layer 210. For example, asillustrated in Examples 8A-8M and Example 8O, the patterned bondinglayer may include a plurality of laterally separate locations of thebonding layer 210. In an embodiment, the plurality of micro p-n diodes135, 150 each include a bottom surface 151 which has approximately thesame or greater width than a corresponding top surface 211 for aplurality of laterally separate locations of the bonding layer 210.

As previously described the bonding layer may absorb compression forcesassociated with contacting the micro LED structure with a transfer headduring the pick up process. As a result, the bonding layer may absorbthe compressive forces and bulge out laterally. Where each micro LEDstructure is patterned to have a small separation distance, of 2 μm forexample, the amount of bonding layer laterally protruding from eachmicro LED structure should be minimized so as to not interfere with anadjacent micro LED structure during the pick up process. In certainembodiments where trenches 206 are present between posts 202, thetrenches may act as bonding layer reservoirs into which molten bondinglayer may flow without interfering with an adjacent micro LED structure.

In some embodiments, the micro LED structures or array of micro LEDstructures of FIG. 8 (as well as the micro LED structures of FIG. 6Example 6I, and FIG. 7 Examples 7′D-7′I after removal of layer 140) arepoised for pick up and transfer to a receiving substrate, for examplewith a transfer head 300 described in more detail with regard to FIGS.14-16. In other embodiments, a thin conformal dielectric barrier layermay be formed of an array of any of the micro p-n diodes 135, 150 priorto pick up and transfer to a receiving substrate. Referring now to FIGS.9′9″, a thin conformal dielectric barrier layer 160 may be formed overan array of any of the micro p-n diodes 150 of FIGS. 7-7″. In oneembodiment, the thin conformal dielectric barrier layer 160 may protectagainst charge arcing between adjacent micro p-n diodes 150 during thepick up process, and thereby protect against adjacent micro p-n diodes150 from sticking together during the pick up process. The thinconformal dielectric barrier layer 160 may also protect the sidewalls153, quantum well layer 116 and bottom surface 151, of the micro p-ndiodes 150 from contamination which could affect the integrity of themicro p-n diodes 150. For example, the thin conformal dielectric barrierlayer 160 can function as a physical barrier to wicking of the bondinglayer material 210 up the sidewalls and quantum layer 116 of the microp-n diodes 150 as described in more detail with regard to FIGS. 11A-11Cin the following description. The thin conformal dielectric barrierlayer 160 may also insulate the micro p-n diodes 150 once placed on areceiving substrate. In an embodiment, the thin conformal dielectricbarrier layer 160 is approximately 50-600 angstroms thick aluminum oxide(Al₂O₃). Conformal dielectric barrier layer 160 may be deposited by avariety of suitable techniques such as, but not limited to, atomic layerdeposition (ALD).

The thin conformal dielectric layer and contact openings can be formedusing a mask layer lift off technique. Referring to FIGS. 9-9′, the masklayer 140 illustrated in FIG. 7 for patterning the micro p-n diode 150can also be used in a lift off technique for forming the thin conformaldielectric barrier layer 160 and contact opening 162. The thin conformaldielectric barrier layer 160 may be formed over an array of any of themicro p-n diodes 150 of FIG. 7, FIG. 7′ or FIG. 7″ and is conformal toand spans across exposed surfaces of the mask layer 140, and sidewalls153 and the bottom surface 151 of the p-n diode 150. The conformaldielectric barrier layer 160 may also span across exposed surfaces ofmetallization layer 120, bonding layer 210, as well as the carriersubstrate and posts 202, if present. The mask layer 140 is then removed,lifting off the portion of the thin conformal dielectric barrier layer160 formed thereon resulting in the structure illustrated in FIG. 9′including contact openings 162. In the particular embodiment illustratedin FIG. 9′, the conformal dielectric barrier layer 160 is not formed onthe top surface 152 of the micro p-n diodes 150.

Referring to FIGS. 10-10″ the thin conformal dielectric layer can alsobe formed over the array of micro p-n diodes 135, 150 of FIG. 8 (as wellas the micro LED structures of FIG. 6 Example 6I, and FIG. 7 Examples7′D-7′I after removal of layer 140) followed by patterning to createcontact openings 162. As illustrated in FIG. 9, the thin conformaldielectric barrier layer 160 may be formed over an array of any of themicro p-n diodes 150 and is conformal to and spans across the exposedtop surface and sidewalls of the p-n diodes 150. The dielectric barrierlayer 160 may also span across the exposed bottom surface 151 of the p-ndiodes 135, 150 and surfaces of metallization layer 120, bonding layer210, as well as the carrier substrate 201 and posts 202, if present. Ablanket photoresist layer may then be formed over the p-n diode arrayand carrier substrate 201, and then patterned to form openings over eachmicro p-n diode 135, 150. The thin conformal dielectric barrier layer160 may then be etched to form contact openings 162 on the top surfaceof each micro p-n diode 135, 150. Contact openings 162 are illustratedin FIGS. 10′-10″ after removal of the patterned photoresist. Asillustrated in FIG. 10′, contact openings 162 may have a slightlysmaller width than the top surface of the micro p-n diodes 135, 150. Thedifference in width may be a result of adjusting for an alignmenttolerance in patterning the photoresist. As a result, the conformaldielectric barrier layer 160 may form a lip around the top surface andsidewalls of the micro p-n diodes 135, 150. As illustrated in FIG. 10″,contact openings 162 may have a slightly larger width than the topsurface of the micro p-n diodes 135, 150. In the embodiment illustratedin FIG. 10″ the contact openings 162 expose the top surfaces of themicro p-n diodes 150 and an upper portion of the sidewalls of the microp-n diodes 150, while the dielectric barrier layer 160 covers andinsulates the quantum well layers 116.

Referring now to FIGS. 11A-11C, in accordance with some embodiments ofthe invention it is possible that an amount of bonding layer 210 wicksup along the side surfaces of the metallization layer 120 and along thebottom surface 151 of the p-n diode layer 110 during the bondingoperation illustrated in FIG. 3. Referring to FIG. 10B, it is possiblethat after forming the micro p-n diodes 150, that the amount bondinglayer 210 which has wicked up could potentially continue its migrationalong the sidewalls 153 of the micro p-n diode 150 during subsequentprocessing. Continued migration toward the quantum well layer 116 couldinterfere with the operation of the micro p-n diode 150. Referring nowto FIG. 10C, in accordance with embodiments of the invention, theconformal dielectric barrier layer 160 may function as a physicalbarrier to protect the sidewalls 153 and quantum well layer 116 of themicro p-n diodes 150 from contamination by the bonding layer material210 during subsequent temperature cycles (particularly at temperaturesabove the liquidus or melting temperature of the bonding layer material210) such as during picking up the micro device from the carriersubstrate, and releasing the micro device onto the receiving substrate.While FIGS. 11A-11C have been illustrated and described with referenceto micro p-n diodes 150, it is also contemplated that it is possiblethat an amount of bonding layer 210 could wick up and continue itsmigration along the sidewalls of micro mesas 130 used to form micro p-ndiodes 135 during the bonding operation illustrated in FIG. 3. Conformaldielectric barrier layer 160 may similarly function as a physicalbarrier to protect the sidewalls and quantum well layer 116 of the microp-n diodes 135 from contamination by the bonding layer material 210.

FIGS. 12A-12B include top and cross-sectional side view illustrations ofa carrier substrate 201 and array of micro LED structures in accordancewith an embodiment of the invention. In the particular embodimentsillustrated, the arrays are produced from micro LED structures ofExample 10′N including micro p-n diode 150. However, it is to beappreciated that FIGS. 12A-12B are meant to be exemplary, and that thearray of micro LED structures can be formed from any of the micro LEDstructures previously described. In the embodiment illustrated in FIG.12A, each individual micro p-n diode 150 is illustrated as a pair ofconcentric circles having different diameters or widths correspondingthe different widths of the top and bottom surfaces of the micro p-ndiode 150, and the corresponding tapered sidewalls spanning between thetop and bottom surfaces. In the embodiment illustrated in FIG. 12B, eachindividual micro p-n diode 150 is illustrated as a pair of concentricsquares with tapered or rounded corners, with each square having adifferent width corresponding to the different widths of the top andbottom surfaces of the micro p-n diode 150, and the correspondingtapered sidewalls spanning from the top and bottom surfaces. However,embodiments of the invention do not require tapered sidewalls, and thetop and bottom surfaces of the micro p-n diode 150 may have the samediameter, or width, and vertical sidewalls. As illustrated in FIGS.12A-12B the array of micro LED structures is described as having a pitch(P), spacing (S) between each micro LED structure and maximum width (W)of each micro LED structure. In order for clarity and conciseness, onlyx-dimensions are illustrated by the dotted lines in the top viewillustration, though it is understood that similar y-dimensions mayexist, and may have the same or different dimensional values. In theparticular embodiments illustrated in FIGS. 12A-12B, the x- andy-dimensional values are identical in the top view illustration. In oneembodiment, the array of micro LED structures may have a pitch (P) of 10μm, with each micro LED structure having a spacing (S) of 2 μm andmaximum width (W) of 8 μm. In another embodiment, the array of micro LEDstructures may have a pitch (P) of 5 μm, with each micro LED structurehaving a spacing (S) of 2 μm and maximum width (W) of 3 μm. However,embodiments of the invention are not limited to these specificdimensions, and any suitable dimension may be utilized.

An embodiment of a method of transferring a micro LED structure to areceiving substrate is described in FIG. 13. In such an embodiment acarrier substrate is provided having an array of micro LED structuresdisposed thereon. As described above, each micro LED structure mayinclude a micro p-n diode and a metallization layer, with themetallization layer between the micro p-n diode and a bonding layer onthe carrier substrate. A conformal dielectric barrier layer mayoptionally span sidewalls of the micro p-n diode. The conformaldielectric barrier layer may additionally span a portion of the bottomsurface of the micro p-n diode, as well as sidewalls of themetallization layer, and bonding layer if present. Then at operation1310 a phase change is created in the bonding layer for at least one ofthe micro LED structures. For example, the phase change may beassociated with heating the bonding layer above a melting temperature orliquidus temperature of a material forming the bonding layer or alteringa crystal phase of a material forming the bonding layer. The micro p-ndiode and metallization layer, optionally a portion of the conformaldielectric barrier layer for at least one of the micro LED structures,and optionally a portion of bonding layer 210 may then be picked up witha transfer head in operation 1320 and then placed on a receivingsubstrate in operation 1330.

A general illustration of operation 1320 in accordance with anembodiment is provided in FIG. 14 in which a transfer head 300 picks upa micro p-n diode, metallization layer, a portion of the conformaldielectric barrier layer for at least one of the micro LED structures,and a portion of bonding layer 210. In the particular embodimentillustrated a conformal dielectric barrier layer has been formed,however, in other embodiments a conformal dielectric barrier layer maynot be present. In some embodiments a portion of bonding layer 210, suchas approximately half, may be lifted off with the micro LED structure.While a specific micro LED structure including micro p-n diode 150 isillustrated, it is understood than any of the micro LED structuresincluding any of the micro p-n diodes 150 described herein may be pickedup. In addition, while the embodiment illustrated in FIG. 14 shows atransfer head 300 picking up a single micro LED structure, transfer head300 may pick up a group of micro LED structures in other embodiments.

Still referring to FIG. 14, in the particular embodiment illustrated thebottom surface of the micro p-n diode 150 is wider than the top surfaceof the metallization layer 120, and the conformal dielectric barrierlayer 160 spans the sidewalls of the micro p-n diode 150, a portion ofthe bottom surface of the micro p-n diode 150 and sidewalls of themetallization layer 120. This may also apply for micro p-n diodes 135.In one aspect, the portion of the conformal dielectric barrier layer 160wrapping underneath the micro p-n diode 135, 150 protects the conformaldielectric barrier layer 160 on the sidewalls of the micro p-n diode 150from chipping or breaking during the pick up operation with the transferhead 300. Stress points may be created in the conformal dielectricbarrier layer 160 adjacent the metallization layer 120 or bonding layer210, particularly at corners and locations with sharp angles. Uponcontacting the micro LED structure with the transfer head 300 and/orcreating the phase change in the bonding layer, these stress pointsbecome natural break points in the conformal dielectric barrier layer160 at which the conformal dielectric layer can be cleaved. In anembodiment, the conformal dielectric barrier layer 160 is cleaved at thenatural break points after contacting the micro LED structure with thetransfer head and/or creating the phase change in the bonding layer,which may be prior to or during picking up the micro p-n diode and themetallization layer. As previously described, in the liquid state thebonding layer may smooth out over the underlying structure in responseto compressive forces associated with contacting the micro LED structurewith the transfer head. In an embodiment, after contacting the micro LEDstructure with the transfer head, the transfer head is rubbed across atop surface of the micro LED structure prior to creating the phasechange in the bonding layer. Rubbing may dislodge any particles whichmay be present on the contacting surface of either of the transfer heador micro LED structure. Rubbing may also transfer pressure to theconformal dielectric barrier layer. Thus, both transferring a pressurefrom the transfer head 300 to the conformal dielectric barrier layer 160and heating the bonding layer above a liquidus temperature of thebonding layer can contribute to cleaving the conformal dielectricbarrier layer 160 at a location underneath the micro p-n diode 135, 150and may preserve the integrity of the micro LED structure and quantumwell layer. In an embodiment, the bottom surface of the micro p-n diode135, 150 is wider than the top surface of the metallization layer 120 tothe extent that there is room for the conformal dielectric barrier layer160 to be formed on the bottom surface of the micro p-n diode 135, 150and create break points, though this distance may also be determined bylithographic tolerances. In an embodiment, a 0.25 μm to 1 μm distance oneach side of the micro p-n diode 135, 150 accommodates a 50 angstrom to600 angstrom thick conformal dielectric barrier layer 160.

A variety of suitable transfer heads can be utilized to aid in the pickup and placement operations 1320, 1330 in accordance with embodiments ofthe invention. For example, the transfer head 300 may exert a pick uppressure on the micro LED structure in accordance with vacuum, magnetic,adhesive, or electrostatic principles in order to pick up the micro LEDstructure.

FIG. 15 is a cross-sectional side view illustration of a bipolar microdevice transfer head which operates according to electrostaticprinciples in order to pick up the micro LED structure in accordancewith an embodiment of the invention. As illustrated, the micro devicetransfer head 300 may include a base substrate 302, a mesa structure 304including a top surface 308 and sidewalls 306, an optional passivationlayer 310 formed over the mesa structure 304 and including a top surface309 and sidewalls 307, a pair of electrodes 316A, 316B formed over themesa structure 304 (and optional passivation layer 310) and a dielectriclayer 320 with a top surface 321 covering the electrodes 316A, 316B.Base substrate 302 may be formed from a variety of materials such assilicon, ceramics and polymers which are capable of providing structuralsupport. In an embodiment, base substrate has a conductivity between 10³and 10¹⁸ ohm-cm. Base substrate 302 may additionally include wiring (notshown) to connect the micro device transfer heads 300 to the workingelectronics of an electrostatic gripper assembly.

FIG. 16 is an illustration of a receiving substrate 400 onto which aplurality of micro LED structures have been placed in accordance with anembodiment of the invention. For example, the receiving substrate maybe, but is not limited to, a display substrate, a lighting substrate, asubstrate with functional devices such as transistors, or a substratewith metal redistribution lines. In the particular embodimentillustrated, each micro LED structure may be placed over a drivercontact 410. A common contact line 420 may then be formed over theseries of micro p-n diodes 135, 150. As illustrated, the taperedsidewalls of the micro p-n diodes 135, 150 may provide a topographywhich facilitates the formation of a continuous contact line. In anembodiment, the common contact line 420 can be formed over a series ofred-emitting, green-emitting or blue-emitting micro LEDs. In certainembodiments, the common contact line 420 will be formed from atransparent contact materials such as indium tin oxide (ITO). In oneembodiment, the plurality of micro LEDs may be arranged into pixelgroups of three including a red-emitting micro LED, green-emitting microLED, and a blue-emitting micro LED.

Still referring to FIG. 16, a close up illustration of a p-n diode 135,150 is provided in accordance with an embodiment of the invention. Inone embodiment, the p-n diode 135, 150 may include a top n-doped layer114 with a thickness of approximately 0.1 μm-3 μm, quantum well layer116 (which may be SQW or MQW) with a thickness less than approximately0.3 μm, and lower p-doped layer 118 with thickness of approximately 0.1μm-1 μm. In an embodiment, top n-doped layer 114 may be 0.1 μm-6 μmthick (which may include or replace bulk layer 112 previouslydescribed). In a specific embodiment, p-n diodes 135, 150 may be lessthan 3 μm thick, and less than 10 μm wide.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming an array of micro LEDstructures which are poised for pick up and transfer to a receivingsubstrate. Although the present invention has been described in languagespecific to structural features and/or methodological acts, it is to beunderstood that the invention defined in the appended claims is notnecessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A method of transferring a micro LED to areceiving substrate comprising: positioning a transfer head over acarrier substrate having an array of micro LED structures disposedthereon, each micro LED structure comprising: a micro p-n diode; and ametallization layer, wherein the metallization layer is between themicro p-n diode and a bonding layer on the carrier substrate; performingan operation to create a phase change in the bonding layer for at leastone of the micro LED structures; picking up the micro p-n diode and themetallization layer for one of the micro LED structures with thetransfer head; and placing the micro p-n diode and the metallizationlayer for the micro LED structure on a receiving substrate.
 2. Themethod of claim 1, wherein the micro LED structure further comprises aconformal dielectric barrier layer spanning sidewalls of the micro p-ndiode; the method further comprising: picking up the micro p-n diode,the metallization layer and a portion of the conformal dielectricbarrier layer for the micro LED structure with the transfer head; andplacing the micro p-n diode, the metallization layer and the portion ofthe conformal dielectric barrier layer for the micro LED structure onthe receiving substrate.
 3. The method of claim 1, wherein the operationis selected from the group consisting of heating the bonding layer abovea liquidus temperature of the bonding layer and altering a crystal phaseof the bonding layer.
 4. The method of claim 1, wherein the operationcomprises heating the bonding layer above a liquidus temperature of thebonding layer.
 5. The method of claim 4, wherein the liquidustemperature of the bonding layer is below 350° C.
 6. The method of claim5, wherein the bonding layer comprises indium.
 7. The method of claim 5,wherein the bonding layer comprises tin.
 8. The method of claim 2,wherein the micro p-n diode comprises a top surface, a bottom surfaceand tapered sidewalls.
 9. The method of claim 8, wherein the bottomsurface is wider than the top surface.
 10. The method of claim 8,wherein the conformal dielectric barrier layer spans a portion of thebottom surface of the p-n diode.
 11. The method of claim 10, wherein theconformal dielectric barrier layer spans sidewalls of the metallizationlayer.
 12. The method of claim 11, wherein the conformal dielectricbarrier layer spans across a portion of the bonding layer adjacent themetallization layer.
 13. The method of claim 12, further comprisingcleaving the conformal dielectric barrier layer.
 14. The method of claim13, wherein cleaving the conformal dielectric barrier layer comprisestransferring a pressure from the transfer head to the conformaldielectric barrier layer.
 15. The method of claim 13, wherein cleavingthe conformal dielectric barrier layer comprises heating the bondinglayer above a liquidus temperature of the bonding layer.
 16. The methodof claim 15, wherein the liquidus temperature of the bonding layer isbelow 350° C.
 17. The method of claim 16, wherein the bonding layercomprises indium.
 18. The method of claim 16, wherein the bonding layercomprises tin.
 19. The method of claim 1, wherein the transfer exerts apick up pressure on the micro LED structure in accordance with vacuumprinciples.
 20. The method of claim 1, wherein the transfer exerts apick up pressure on the micro LED structure in accordance withelectrostatic principles.